The present invention relates to an image formation method and an image formation apparatus such as a copier, a printer and a facsimile.
Recently, mass colored documents are handled even in an office and accordingly smaller and faster image formation apparatus such as full-color printers and full-color copiers are desired increasingly more than before. A recent widespread color laser printer is generally provided with a plurality of developing devices arranged touchable with one photosensitive member. Each of different colored toner images is formed per revolution of the photosensitive member and the images are transferred in turn from the photosensitive member to a recording material such as a recording paper and an OHP sheet to form a colored toner image. This printer is the so-called one-drum type of mainstream.
The one-drum image formation apparatus are classified into an intermediate transfer type and a direct transfer type. The former is configured to primarily transfer toner images of different colors from a photosensitive member and superimpose them on an intermediate transfer member, then secondarily transfer the toner images together onto a recording material. The latter is configured to transfer toner images of different colors from a photosensitive member sequentially onto a recording paper held on a transfer drum and the like to form a colored toner image.
The image formation apparatus of the direct transfer type has an advantage because of a simple structure, low cost and ability of high-speed image formation. The direct transfer type, however, is difficult to achieve stable transfers because resistances and water contents of the recording paper vary during sequential transfers of different color toners to the recording paper. In contrast, the image formation apparatus of the intermediate transfer type can achieve stable transfers because the transfer of the toner image to the recording paper is required only once. If the image formation apparatus of the direct transfer type employs a transfer drum which wraps a recording material thereon, it has a limitation in a thickness and type of the recording material. To the contrary, the image formation apparatus of the intermediate transfer type has an advantage because it has no limitations in thickness and types of recording materials and is possible to increase their versatility.
In the case of formation of a full-color image from four toner images superimposed, for example, either type of the one-drum image formation apparatus is required to rotate the photosensitive member at least four turns to form different color toner images. This is disadvantageous because formation of a color image requires a long time, which lowers a yield.
In order to respond to high-speed color image formation, plural (three or four in general) photosensitive members are arranged along a path which conveys a recording material. Plural developing devices are respectively mounted on the plural photosensitive members to develop latent images of different colors formed individually on the photosensitive members into toner images, which are transferred sequentially to and successively superimposed on the recording material conveyed to form a color image. Such the configuration can be found in an image formation apparatus of the so-called tandem or inline type proposed in the art. For example, Japanese Patent Application Laid-Open No. 53-74037 (U.S. Pat. No. 4,162,843) proposes an image formation apparatus, which comprises plural photosensitive members mounted thereon for sequentially multiple-transferring toner images on a recording material that is conveyed using a belt-like conveyer unit.
The image formation apparatus of the tandem or inline type is possible to form an image at a higher speed four times or more than that of the image formation apparatus of the one-drum type if the photosensitive members have the same speed on the perimeter. The image formation apparatus of the tandem or inline type is generally configured to transfer the toner image from the photosensitive member directly onto a recording material. This configuration causes several problems because of unstable transferring of the toner image onto the recording material and difficult positioning of the recording material at the time of transferring the toner image, as described above. An image formation apparatus of the so-called tandem intermediate transfer type is proposed to solve such problems, which belongs to the tandem type and employs an intermediate transfer member (see Japanese Utility Model Application Laid-Open No. 59-192159, for example).
The image formation apparatus of such the type is configured to form toner images of different colors on photosensitive members or image carriers using plural developing devices that contain different color toners therein. The toner images are transferred to and superimposed on an intermediate transfer member or a recording material. Then, different color toners not transferred and resided on the surfaces of the photosensitive members are removed and collected by a cleaning device which prepares the next image to be formed on the surfaces of the photosensitive members. The cleaning device may have a configuration for removing and collecting the non-transferred different color toners from the photosensitive members without mixing them. In this case, the different color toners removed and collected by the cleaning device from the photosensitive members can be reused as recycled toners when the collected toner is returned to the corresponding developing device that contains the same color toner.
A color image formation apparatus of the tandem type comprises plural (three or four in general) image formation units each including photosensitive members, developing devices and cleaning devices by the number corresponding to the different color toners required for full-color image formation. Each image formation unit is configured to individually form a toner image of each color. Accordingly, in this color image formation apparatus of the tandem type, non-transferred toner resided on each photosensitive member can be collected by the cleaning device corresponding to each photosensitive member without causing color mixture. Therefore, each color toner collected by each cleaning device can be returned to each developing device and easily reused as a recycled toner.
Recycling of the collected toner is particularly important in the image formation apparatus which forms color images as described above. The image formation apparatus of this type is often employed to create an image with a larger image area rate and a larger number of colors compared to a monochromic image, such as a photographic draft, and accordingly consumes relatively larger amounts of color toners. It is difficult, however, to reserve a storage space for housing sufficient amounts of color toners as the apparatus is downsized and weight reduced. Therefore, it is an important point to recycle the collected toner from the viewpoint of reducing its resource, space and running cost.
The conventional image formation apparatus has the following problem, however, if the collected toners of plural colors are employed as recycled toners.
In such the image formation apparatus that employs plural color toners to form an image, a different color toner may be mixed in the collected toner of each color that is removed and collected by the cleaning device from the photosensitive member. In this case, when the collected toner is reused as a recycled toner, influence from the color mixture may reduce the image quality.
The color mixture of the collected toner occurs in any types of image formation apparatus. In particular, the one-drum image formation apparatus is generally configured to create toner images of different colors sequentially on one photosensitive member and collect residual toners of different colors resided on the photosensitive member using one cleaning device. Therefore, the color mixture of the collected toner can not be avoided. The residual toners of different colors can be collected using plural cleaning devices corresponding to the different colors. In this configuration, similar to the image formation apparatus of the tandem type, extreme color mixture of the collected toner can be avoided. Even in such the configuration, however, the color mixture of the collected toner can not be avoided.
For example, four developing devices are employed to accommodate toners of four colors including yellow, magenta, cyan and black individually. Four toner images are formed in order of yellow, magenta, cyan and black on one or four photosensitive members used as image carriers. The color toner images formed on the photosensitive members are primarily transferred to and superimposed on an intermediate transfer member. Then, the toner images on the intermediate transfer member are secondary transferred integrally onto a recording material. Residual color toners resided on the photosensitive members after the primary transfer are collected individually on a color toner basis using four cleaning devices corresponding to the residual color toners. This case will be considered on the assumption that no toner is resided on the intermediate transfer member after the secondary transfer.
In the above case, after a toner image of the first color or yellow is transferred to the intermediate transfer member, other color toners can not be mixed in the toner resided on the photosensitive member. Therefore, if the residual toner of the first color is collected, the collected toner can be reused directly as a recycled toner without reduction in image quality.
If the toner images of the second and lower orders including magenta, cyan and black are primarily transferred sequentially to the intermediate transfer member, however, other color toners may be mixed in the toners resided on the photosensitive members.
In the image formation apparatus of such the type, when a toner image of the second color or magenta is transferred to the intermediate transfer member, the yellow toner already primarily transferred on the intermediate transfer member is inversely transferred to the surface of the photosensitive member. In this case, the toner of the first color or yellow inversely transferred may be mixed in the residual toner consisting of the toner of the second color or magenta resided on the photosensitive member. Similarly, the toners of the first color or yellow and the second color or magenta inversely transferred to the photosensitive member may be mixed in the residual toner consisting of the toner of the third color or cyan resided on the photosensitive member. The toners of the first color or yellow, the second color or magenta and the third color inversely transferred to the photosensitive member may be mixed in the residual toner consisting of the toner of the fourth color or black resided on the photosensitive member.
From the above reason, with respect to the residual toners of the second and lower orders including magenta, cyan and black, the reuse of their collected toners directly as recycled toners may possibly reduce the image quality.
Several countermeasures have been proposed for preventing such the mixture of the collected toner color. For example, Japanese Patent Application Laid-Open No. 2000-242152 discloses xe2x80x9cColor image formation apparatusxe2x80x9d. In this apparatus, the discharge during transferring a toner image from an image carrier to an intermediate transfer member imparts a charged polarity opposite to the normal charged polarity on a part of the toner previously transferred to the intermediate transfer member. The toner with the opposite charged polarity is inversely transferred to the image carrier when a toner of the next color is transferred. From this consideration, such a collecting unit is provided that utilizes the fact that the inversely transferred toner has the opposite charged polarity. This collecting unit is effective to prevent mixture of colors from occurring on the image carrier.
The above color image formation apparatus, however, requires a unit which removes the opposite charged toners newly located in the vicinity of the photosensitive member. Such the unit has not been required in the art and causes a disadvantage because of an installation space to be reserved and cost-up due to increased components. As for the opposite charged toner, the normal charged polarity is originally stable and an amount of charge on the toner with the opposite polarity is unstable. The toners inversely transferred on the photosensitive member may often include toners charged almost xe2x80x9czeroxe2x80x9d. Therefore, the unit which removes the opposite charged toners described above is not possible to collect the color-mixed toners completely. This method is thus not effective to reliably prevent mixture of colors from occurring in the collected toner.
Japanese Patent Application Laid-Open No. 10-293432 discloses xe2x80x9cImage formation apparatusxe2x80x9d. This apparatus is not the tandem type but adopts a method of erasing the background potential, prior to the transferring, to prevent the opposite charging of the toner that causes the toner to be inversely transferred to the photosensitive member. The extent of the erasing is adjusted to prevent the toner image from scattering before the transferring. In practice, however, the light illumination for erasing reduces the potential sharply and makes the toner on the image section move to the periphery, resulting in a fogged image.
If the background potential before the transferring is erased insufficiently, due to the discharge during transferring the toner image from the image carrier to the intermediate transfer member, the opposite charged toner is inversely transferred to the photosensitive member. This case is similar to xe2x80x9cColor image formation apparatusxe2x80x9d disclosed in Japanese Patent Application Laid-Open No. 2000-242152. Therefore, the color mixture of the collected toner due to the opposite charged toner inversely transferred to the photosensitive member can not be prevented. Thus, this image formation apparatus is extremely difficult to completely prevent the color mixture of the collected toner from occurring.
Japanese Patent Application Laid-Open No. 8-63067 and 2000-267366 entitled xe2x80x9cImage formation apparatusxe2x80x9d describe a problem related to the toner collected by cleaning that causes aggregation of toner and reduction in charging property and propose a supply ratio of a recycled toner to be determined to correct the process condition. This image formation apparatus can not detect or predict a state of color mixture of the collected toner while paying attention to the color mixture of the collected toner as described above. When the apparatus reuses the collected toner as a recycled toner, it can not suppress reduction in image quality due to the color mixture of the collected toner.
It is an object of this invention to provide an image formation method and an image formation apparatus using the same capable of suppressing reduction in image quality due to color mixture of the toner collected from an image carrier when the collected toner is reused as a recycled toner.
The present invention has another object to provide an image formation method and an image formation apparatus using the same for reusing a color-mixed recycled toner independent of a degree of color mixture. This makes it possible to adjust an amount of the recycled toner relative to a new toner and control a variation in color reproduction in an image formed.
According to one aspect of the present invention, an image formation method comprises a latent image formation step of forming a plurality of latent images, sequentially on an image carrier, corresponding to a plurality of image colors for forming an image, a development step of supplying a plurality of specific color toners corresponding to the image colors, onto the formed latent images, from a plurality of developing devices holding the plurality of specific color toners therein, to develop the formed latent images into toner images, a image formation step of sequentially transferring the toner images to be superimposed on a recording material to form a multicolored image, the recycle step of collecting specific color toners resided after transferring the toner images on the image carrier and reusing the collected toners once returned to the developing devices as recycled toners, and a control step of controlling a mixture ratio of the recycled toner to the specific color toner in each of the developing devices below a limiting color mixture ratio, which is defined as a mixture ratio of a recycled toner to a specific color toner in a developing device when a tone variation in image is still on a permissible limitation level, which tone variation is caused from mixture of the recycled toner into the specific color toner in the developing device at the recycle step.
According to another aspect of the present invention, an image formation method comprises charging and exposing surfaces of a plurality of image carriers to form electrostatic latent images thereon, attaching specific color toners corresponding to the image carriers onto the electrostatic latent images to develop the electrostatic latent images, transferring the developed images to be superimposed on an intermediate transfer member to form toner images on the intermediate transfer member, transferring the toner images sequentially on a recording sheet, cleaning the image carriers to remove specific color toners resided thereon using cleaning devices contained in the image carriers, and forming toner images on the intermediate transfer member at the second or lower transfer order using recycled toners collected by the cleaning devices together with new toners.
According to still another aspect of the present invention, an image formation apparatus comprises an image carrier which carries a plurality of latent images formed thereon corresponding to different colors in a colored image of at least two colors, a plurality of developing devices which supplies specific color toners corresponding to image colors to develop the latent images formed on the image carrier, a transferring unit which sequentially transfers color toner images, developed on the image carrier using the specific color toners in the developing devices, to be superimposed on a recording sheet, a cleaning unit which collects toners not transferred and resided on the image carrier after transferring the toner images, a toner conveying unit which returns the toners collected by the cleaning unit to the developing devices, and a control unit which controls a mixture ratio of the recycled toner to the specific color toner in each of the developing devices using the above image formation method.
According to still another aspect of the present invention, an image formation apparatus comprises a plurality of image carriers, charging and an exposing unit which forms electrostatic latent images on the image carriers based on image signals, a developing unit which attaches specific color toners to the electrostatic latent images on the image carriers to develop the electrostatic latent images into toner images, a transferring unit which transfers the toner images sequentially onto an intermediate transfer member, a cleaning unit which removes toners attached on the intermediate transfer member after the transferring, a path unit which returns recycled toners collected by the cleaning unit to the developing unit at the time of transferring toner images on the intermediate transfer member at the second or lower transfer order, and a path unit which supplies new toners to the developing unit.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.